1. Field of the Invention
The present invention generally relates to electrical circuits for supplying energy to multiple loads, such as CCFL (Cold-Cathode-Fluorescent-Lamp) loads and more particularly to electrical circuits for providing phase shifts to multiple loads. Usually, the electrical circuits are applied to display devices, such as liquid crystal display monitors, liquid crystal display computers or liquid crystal display televisions.
2. Description of the Related Art
CCFL loads are extensively used to provide backlighting for liquid crystal displays (LCD), particularly for backlighting LCD monitors and LCD televisions. However, such conventional applications require a separate direct current/alternative current power inverter (DC/AC power inverter) to drive an individual CCFL. Such an application is illustrated in FIG. 1, wherein each CCFL (20, 22, . . . 24) is powered by an individual DC/AC inverter (10, 12, . . . 14), respectively, and all DC/AC inverters are synchronized. Each DC/AC inverter includes a switched AC network and a power driver circuit. The power driver circuit may include a resonant tank circuit for the CCFL. The switched AC network in each inverter is driven ON/OFF synchronously. Therefore, there is a large ripple on the power line. A large current will be drawn from the power source VBatt when the switches in the switched networks are turned on and the current drawn is released when the switches are turned off. The simultaneous turning on and off at all inverters cause noises on the power line which degrades the signal/noise integrity in the system.
One method to reduce the ripple is to increase the filtering at the power line. However, the disadvantage is that the size of the circuit is increased, which in turn increases the system cost.
FIG. 2 shows another prior art circuit for driving multiple CCFL loads where the circuit includes a controller (40) for driving multiple DC/AC inverters (10, 12, 14, . . . 16) and CCFL loads (20, 22, 24, . . . 26). A clock generator (42) in the controller (40) generates a string of phase-shifted clock signals to each DC/AC inverter (10, 12, 14, . . . 16) to make a phase delay. Since the switches in the networks o f a 11 D C/AC inverters (10, 12, 14, . . . 16) are turned on and turned off with equal phase shift between the adjacent inverters, the ripple on the power line is effectively reduced to 1/N of that shown in FIG. 1, where N is the number of DC/AC inverters connected.
However, the problem is that the controller (40) is fixed to the number of original demanded loads, in other words, the number of CCFL loads equals to the lines which deliver phase shifts from the controller (40) to each inverter (10, 12, 14, . . . 16). Therefore, if the number of CCFL loads is changed, the configuration of the controller (40) should be changed. Another disadvantage is that the controller (40) needs to generate a high-frequency clock signal having a frequency of N times the operating frequency of the individual DC/AC inverter.
It is an object of the present invention to provide a simpler controller of a simpler phase shift circuit technique, wherein a simpler user-programmable number of phases according to the number of inverters is used, such that a high power, low cost and smaller multiple-inverter system can be achieved.
It is another object of the present invention to provide an improved electrical circuit comprising multiple power circuits, such as DC/AC inverters, for driving multiple loads, such as CCFL loads, which reduces instantaneous high current ripples and noises caused by turning ON and turning OFF switches in the power circuits simultaneously.
It is yet another object of the present invention to provide a display device comprising at least two loads, such as CCFL loads. The display device may be an LCD monitor, an LCD television or an LCD computer.
It is yet another object of the present invention to provide a method for driving an electrical circuit comprising multiple power circuits, such as DC/AC inverters, for driving multiple loads, such as CCFL loads, which reduces instantaneous high current ripples and noises caused by turning ON and turning OFF switches in the power circuits simultaneously.
Briefly, the controller of the present invention comprises a pulse generator for generating a clock signal for initiating the operation of the DC/AC inverters connected; and a phase selector for generating a reference signal indicating a number of DC/AC inverters connected.
In addition, the electrical circuit in accordance with the present invention provides a phase shift of the switching ON/OFF between DC/AC inverters for driving CCFL loads, wherein the number of phase shifts is programmed in accordance with the number of DC/AC inverters connected. According to the present invention, a first DC/AC inverter receives a first pulse signal from the pulse generator and a reference signal from the phase selector indicating the number of phases and generates a second pulse signal to a second DC/AC inverter. The second DC/AC inverter receives the second pulse signal having a phase shift with respect to the first pulse signal input to the first DC/AC inverter. Likewise, the second DC/AC inverter generates a third pulse signal to a third DC/AC inverter having the same amount of phase shift with respect to the second pulse signal input to the second DC/AC inverter, whereby all DC/AC inverters are turned on and turned off with equal phase shift between the adjacent inverters. According to the present invention, the ripple on the power line is thus effectively reduced, the circuit is programmable and simplified and the cost is reduced.